The present invention relates to connectors for mounting integrated circuit packages to circuit boards. More particularly, the present invention relates to a micro-miniature resilient support for supporting an integrated circuit chip or multiple-chip module (MCM) on a circuit board, and for interconnecting electrical circuits on a chip or multiple-chip module with circuits on the circuit board.
In the past, leadless ceramic chip carriers (LCCCs) exhibited problems when exposed to harsh thermally cyclic environments and vibration. Due to the thermal cycles and vibration, the solder joints which connected the LCCC to a circuit board degraded and cracked over time. The degradation and cracking was caused by stresses and strains which are induced at the solder joint by temperature cycling. Ultimately, the solder joints failed both mechanically and electrically causing failures in the electronic system in which they were used.
As a result, a compliant electrical interface between the leadless chip carrier and the circuit board, suitable for mass production using current surface mount technology, has been developed. Such an interface is described in greater detail in the Pai et al U.S. Pat. No. 4,827,611 which is hereby fully incorporated by reference. The curved leads described in the Pai et al patent serve as an electrically conductive path between the LCCC and the circuit board. The curved leads also form a mechanically compliant interface that withstands the damaging thermal stresses and vibration which degraded and destroyed solder joints in previous interfaces.
Although the compliant interface described in the Pai et al patent provides an effective interface between the LCCC and the circuit board, it is subject to certain problems. The curved leads in the Pai et al patent include tangs which carry a solder slug or bead. During a soldering process, heat is applied to the solder of the solder slug causing the solder to flow to form the solder connection between the lead and the LCCC. The flowing solder tends to run down the curved lead into the bent portions of the lead. Then, when the heat is removed and the solder cools, it hardens within the bent portions of the curved lead. This hardened solder reduces compliancy of the curved lead thereby making the interface formed by the lead less effective.
A second problem with the solder slug used on the leads in the Pai et al patent is their cost. Forming the tangs and placing the solder slug within the tangs on the curved leads is a costly assembly step.
Another problem is that if too much solder is included in the solder slug, the solder runs beyond the area intended as a solder joint. This excess solder creates adverse signal characteristics and, in severe cases, short circuits with adjacent leads or traces on the circuit board. Consequently, the circuit board must be carefully cleaned after solder connection. This is a difficult and expensive process.
In addition, the tangs and solder slugs at the end of the curved leads were relatively large in comparison to the lead width. Hence, additional space was required between adjacent leads limiting lead density.
Another problem resulted from a two step assembly process used to mount the LCCCs to a circuit board. The leads would be soldered first to the LCCC and then to the circuit board. Alignment problems resulted during the second soldering step of soldering the leads to the circuit board.
In the past, it was also widely thought that merely solder plating the leads would be ineffective in providing adequate solder joints in such applications. It was thought that gravitational forces would pull the molten solder, which had been plated on the lead, away from the solder joint during soldering thereby providing insufficient solder to form the required joint. Thus, the tangs and solder beads of the prior art tended to be the trend in manufacturing such devices.